Here's what NYSE trading looked like before and after the outage

As yet, no one knows why. The NYSE has said that's it's not a
hack though.

"The issue we are experiencing is an internal technical
issue and is not the result of a cyber breach," the
NYSE tweeted
out about an hour after the halt. "We
chose to suspend trading on NYSE to avoid problems arising from
our technical issue. NYSE-listed securities continue to
trade unaffected on other market centers."

Software experts tell Business Insider that this could be a
problem with NYSE's stock-pricing program, similar to what
happened at the NASDQ in August 2013.